Clarify documentation around ordering and add test

Author: joto

include/osmium/osm/object_comparisons.hpp

@@ -154,15 +154,17 @@ namespace osmium {
      * (negative IDs first, then positive IDs, both in the order of their
      * absolute values), but later versions of an object are ordered before
      * earlier versions of the same object. This is useful when the last
-     * version of an object needs to be used.
+     * version of an object needs to be used. Object visibility is not taken
+     * into account, so objects with different visibilities keep their order.
      */
     struct object_order_type_id_reverse_version {
 
         bool operator()(const osmium::OSMObject& lhs, const osmium::OSMObject& rhs) const noexcept {
+            const bool tsvalid = lhs.timestamp().valid() && rhs.timestamp().valid();
             return const_tie(lhs.type(), lhs.id() > 0, lhs.positive_id(), rhs.version(),
-                        ((lhs.timestamp().valid() && rhs.timestamp().valid()) ? rhs.timestamp() : osmium::Timestamp())) <
+                        (tsvalid ? rhs.timestamp() : osmium::Timestamp())) <
                    const_tie(rhs.type(), rhs.id() > 0, rhs.positive_id(), lhs.version(),
-                        ((lhs.timestamp().valid() && rhs.timestamp().valid()) ? lhs.timestamp() : osmium::Timestamp()));
+                        (tsvalid ? lhs.timestamp() : osmium::Timestamp()));
         }
 
         /// @pre lhs and rhs must not be nullptr

test/t/osm/test_object_comparisons.cpp

@@ -195,6 +195,19 @@ TEST_CASE("Node comparisons") {
         REQUIRE(std::is_sorted(nodes.cbegin(), nodes.cend(), osmium::object_order_type_id_reverse_version{}));
     }
 
+    SECTION("reverse version ordering should keep order for deleted objects") {
+        nodes.emplace_back(buffer.get<osmium::Node>(osmium::builder::add_node(buffer, _id( 1), _version(2), _timestamp("2016-01-01T00:00:00Z"), _deleted(false))));
+        nodes.emplace_back(buffer.get<osmium::Node>(osmium::builder::add_node(buffer, _id( 1), _version(2), _timestamp("2016-01-01T00:00:00Z"), _deleted(true))));
+
+        REQUIRE(std::is_sorted(nodes.cbegin(), nodes.cend(), osmium::object_order_type_id_reverse_version{}));
+
+        nodes.clear();
+
+        nodes.emplace_back(buffer.get<osmium::Node>(osmium::builder::add_node(buffer, _id( 1), _version(2), _timestamp("2016-01-01T00:00:00Z"), _deleted(true))));
+        nodes.emplace_back(buffer.get<osmium::Node>(osmium::builder::add_node(buffer, _id( 1), _version(2), _timestamp("2016-01-01T00:00:00Z"), _deleted(false))));
+
+        REQUIRE(std::is_sorted(nodes.cbegin(), nodes.cend(), osmium::object_order_type_id_reverse_version{}));
+    }
 }
 
 TEST_CASE("Object comparisons: types are ordered nodes, then ways, then relations") {